Chemical sensors are devices that detect the presence and/or level of a particular chemical species (an “analyte”) in the air, water, or another medium. There is a high demand for chemical sensor devices able to detect low concentration levels of analytes in the liquid and/or gaseous phases. Specificity to particular analytes (the ability of a sensor to distinguish one species from another) is also generally desired.
Chemical sensor devices often involve luminescent materials because luminescence lifetimes and intensities can be sensitive to the presence of an external analyte. A typical sensor of this type includes a binding site where an analyte binds to a composition or precursor on the sensor, and this binding affects the intensity and/or wavelength of luminescence of a luminescent material in the sensor in a manner that is observable. Fluorescent materials are one class of luminescent materials that are particularly useful for sensor devices because their fluorescence and/or other physical properties can be optimized and/or tailored for particular analytes through chemical structure changes of the material, i.e., their sensitivity to a particular analyte can be controlled. Fluorescent materials are able to fluoresce in some cases because they are capable of delocalizing electronic charge throughout a substantial portion of the material through electron pi-conjugation, via a pi-conjugated portion comprising a set of orbitals that can function as a valence band. The energy difference between the valence band and non-valence or conduction bands of the material is generally referred to as a “band gap,” and energy transitions of electrons between these levels can cause fluorescence. Other energy levels of the material may also be available in the band gap or in higher energy levels having antibonding character.
Because electronic charge delocalization through pi-conjugation of fluorescent materials results in the formation of various elevated energy levels, a variety of excited state structures are available upon absorption of external excitation energy by the fluorescent material. The luminescence yields of these excited state structures generally depends on the structure. The luminescence of some materials can be “quenched” by the presence of an analyte capable of absorbing the excitation energy contained by the material, resulting in the material returning to a ground state without causing luminescence. The analyte can be externally or internally located within the material. One example of such quenching is through a pi-stacking mechanism. In a pi-stacking mechanism, atoms involved in pi-conjugation can be positioned on top of other moieties having geometrically accessible pi-orbitals, thereby forming a pathway for energy transfer that allows quenching to occur.